Ultrafast in situ microwave-assisted hydrothermal synthesis of nanorods and soft magnetic colloidal nanoparticles based on MnFe2O4

This work presents for the first time one-step ultrafast (precursor-free) synthesis of 1D MnFe2O4 (MFO) nanorods and soft magnetic colloidal nanoparticles (NPs) using microwave-assisted hydrothermal (MAH) methods, with or without citric acid (CA) as a surfactant (in situ synthesis), respectively. The mechanism of growth of spinel MFO nanostructures during the MAH synthesis was studied by varying synthesis duration (3–6 h) and temperature (180–200 °C). An increase in both the duration and temperature improved the purity of the samples, up to 97%. On the other hand, a temperature increase by 20 °C notably shortened the formation time of MFO nanorods, which have an average diameter and length of less than 20 nm and 350 nm, respectively, as observed at 200 °C after 6 h. All the fabricated MFO NPs with spherical and rod-like morphologies manifested high saturation magnetization in the range of 54–64 emu/g. The chelation of lattice metal ions by CA resulted in the formation of a stable colloid comprising 100% pure spinel MFO NPs with a size of ≤32 ± 10 nm (mean ± SD) and featuring very soft magnetic properties. This colloid was generated by the MAH synthesis at 175 °C within 30 min. Notably, an increase in synthesis duration from 30 min to 3 h diminished MFO phase purity from 100% to 52% and saturation magnetization from 43.4 ± 0.7 to 33.9 ± 2.0 emu/g for CA-functionalized MFO NPs owing to CA degradation increasing during the in situ MAH synthesis with longer duration. This study indicates good potential of ultrafast MAH synthesis for the development of 1D magnetic spinel nanostructures with controllable morphology, size, magnetic properties, and colloidal stability, thereby offering a wide range of applications within the fields of adsorption, catalysis, electronics, and biomedicine.